This invention relates to the structure of an electromagnetic transducer, in particular, relates to an improved structure of a voice coil motor, which means, in the present specification, a linear motor working according to the principle of a dynamic sound speaker.
A voice coil motor, according to the present invention, may be applied, for instance, to a linear driving motor which positions in a short time movable magnetic heads or flying heads on the desired track of a magnetic medium of a magnetic disc unit in an information processing system such as a computer system. The access time that a central processing unit of a computer system must wait to read or write desired information to or from a magnetic disc unit should be as short as possible, preferably less than 60 mS. And the access time of a magnetic disc unit depends mainly on the positioning time of movable heads. Therefore, the characteristics of a linear driving motor for positioning movable heads affects not only the access time of a magnetic disc unit, but also the performance of the whole computer system.
The linear driving motor for positioning movable heads has been provided with an hydraulic actuator, a printed circuit motor or a pulse motor in which the access time of a magnetic disc unit takes 70 - 80 mS on the average. However, this access time is so long for a high speed computer system that various kinds of motors which provide shorter access time have been proposed and developed. One of the motors which has been developed for positioning movable heads is a voice coil motor which works on the principle of a dynamic sound speaker.
One disadvantage of the prior voice coil motor is that flux density in a magnetic pole gap of a voice coil motor is insufficient to obtain enough force or torque for positioning movable heads in a short time. Another disadvantage of the prior voice coil motor is that the inductance of a moving coil is too large and, therefore, high speed positioning and stable control of movable heads are not obtained.
In order to overcome the above disadvantages, a voice coil motor has been developed which has a cylindrical smooth magnetic pole inscribed on a ferrite magnet. Although this motor overcomes a part of the above disadvantages, additional improvement is still necessary.
Scarrott disclosed an improved motor by providing a laminated magnetic pole on a smooth magnetic pole in U.S. Pat. No. 3,694,678. However, Scarrott's motor has only a small effect for reducing the inductance of a moving coil, since the laminated magnetic pole is not directly connected to a permanent magnet, but a smooth magnetic pole is provided between the lamination and the permanent magnet. The magnetic pole must be completely laminated or toothed, otherwise the magnetic flux goes through the untoothed portion of the magnetic pole and the inductance can not be decreased very much since the material of the magnetic pole has considerably large permiability .mu.. However, due to the special structure of Scarrott's motor, the magnetic pole can not be completely toothed.